1. Field of the Invention
The present invention relates to liquid crystal display devices and more particularly to failure detection and testing of a liquid crystal display.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) displays an image by adjusting light transmission of liquid crystal cells arranged in a matrix on a liquid crystal display panel according to video signals.
The liquid crystal display panel includes a thin film transistor (TFT) substrate and a color filter substrate bonded together by a sealant with liquid crystal interposed therebetween.
In the color filter substrate, a color filter array including a black matrix for preventing light leakage, a color filter for realizing color, a common electrode for generating a vertical electric field with a pixel electrode, and an upper alignment film that is coated thereon for aligning the liquid crystal is formed on an upper substrate.
In the TFT substrate, a TFT array includes a gate line and a data line formed to cross each other, a TFT formed at a crossing thereof, a pixel electrode connected to the TFT, and a lower alignment film that is coated thereon for aligning the liquid crystal is formed on a lower substrate. In particular, a gate driver may be simultaneously formed in a common process with the TFT in a peripheral region of the TFT substrate. An amorphous silicon TFT or a polysilicon TFT having high charge mobility is used in the TFT formed in the gate driver.
A typical method of manufacturing the liquid crystal display panel includes a patterning process for forming the TFT array and the color filter array, a sealing process for bonding the TFT substrate and the color filter substrate with liquid crystal interposed therebetween, and a testing process for detecting a faulty liquid crystal display panel.
In the testing process, the TFT is turned on in response to a scan signal generated by the gate driver and a test pixel signal is supplied from the data line to the liquid crystal cell via the TFT, such that a failure of the liquid crystal display panel is detected. When a failure occurs in a signal line of the liquid crystal display panel, a pixel connected to the signal line displays an image different from a pixel connected to a normal signal line and accordingly a failure state of the liquid crystal display panel can be easily detected.
However, using the related art methods of testing the liquid crystal display, it is not impossible to detect some failures of the gate driver that may occur while the gate driver is manufactured. That is, when a failure occurs in the gate driver, the gate driver may abnormally operate to output a scan signal having an abnormal level instead of a normal level the failure may not be detected. For example, when clock lines for supplying a plurality of clock signals to the gate driver are slightly short-circuited, a scan signal having an abnormal level between a low logic level and a high logic level may be generated by a stage that should generate a scan signal having a low logic level. The TFT operates in response to the scan signal having the abnormal level and thus the liquid crystal display panel is determined to be good. In the liquid crystal display panel that is determined to be good in spite of having a scan signal having the abnormal level, a failure phenomenon may occur in a driving environment or upon application of a specific driving pattern. The failure phenomenon may gradually become more serious so that a defect becomes visible to a user after the elapse of the time.